1. Field of the Invention
The present invention relates to a hard film to improve the wear resistance of cutting tools such as tips, drills, and end mills, a cutting tool coated with said hard film which exhibits excellent wear resistance, a process for forming said hard film, and a target used as a vapor source to form said hard film.
2. Description of the Related Arts
It has been common practice to coat cutting tools made of cemented carbide, cermet, or high speed tool steel with hard film of TiN, TiCN, TiAlN, or the like for the purpose of improving their wear resistance.
Because of its excellent wear resistance as disclosed in Japanese Patent No. 2644710, the film of compound nitride of Ti and Al (referred to as TiAlN hereinafter) has superseded the film of titanium nitride, titanium carbide, or titanium carbonitride to be applied to cutting tools for high speed cutting or for high hardness materials such as quenched steel.
There is an increasing demand for hard film with improved wear resistance as the work material becomes harder and the cutting speed increases.
It is known that the above-mentioned TiAlN film increases in hardness and improves in wear resistance upon incorporation with Al. Japanese Patent No. 2644710 indicates that TiAlN precipitates soft AlN of ZnS structure when the Al content therein is such that the compositional ratio x of Al exceeds 0.7 in the formula (AlxTi1xe2x88x92x)N representing TiAlN. The foregoing patent also mentions that xe2x80x9cif the Al content (x) exceeds 0.75, the hard film has a composition similar to that of AlN and hence becomes soft, permitting the flank to wear easilyxe2x80x9d. In addition, the foregoing patent shows in FIG. 3 the relation between the compositional ratio of Al and the hardness of film. It is noted that the hardness begins to decrease as the compositional ratio of Al exceeds about 0.6. This suggests that AlN of ZnS structure begins to separate out when the compositional ratio of Al is in the range of 0.6-0.7 and AlN of ZnS structure separates out more as the compositional ratio of Al increases further, with the result that the hardness of film decreases accordingly. Moreover, the foregoing patent mentions that the TiAlN film begins to oxidize at 800xc2x0 C. or above when the compositional ratio x of Al is 0.56 or higher, and this temperature rises according as the value x increases. The temperature which the TiAlN film withstands without oxidation is about 850xc2x0 C. when the compositional ratio of Al is 0.75 (which is the upper limit for the TiAlN film to have adequate hardness).
In other words, the conventional TiAlN film cannot have both high hardness and good oxidation resistance because there is a limit to increasing hardness by increasing the compositional ratio of Al. Consequently, it is limited also in improvement in wear resistance.
At present, cutting tools are required to be used at higher speeds for higher efficiency. Cutting tools meeting such requirements need hard coating film which has better wear resistance than before.
The present invention was completed in view of the foregoing. It is an object of the present invention to provide a hard film for cutting tools which is superior in wear resistance to TiAlN film and permits high-speed efficient cutting, a process for forming said hard film, and a target used to efficiently form a hard film for cutting tools by said process.
The present invention is directed to a hard film for cutting tools composed of
(Ti1xe2x88x92axe2x88x92bxe2x88x92cxe2x88x92d, Ala, Crb, Sic, Bd(C1xe2x88x92aNe) 0.5xe2x89xa6axe2x89xa60.8, 0.06xe2x89xa6b, 0xe2x89xa6cxe2x89xa60.1, 0xe2x89xa6dxe2x89xa60.1, 0xe2x89xa6c+dxe2x89xa60.1, a+b+c+d less than 1, 0.5xe2x89xa6exe2x89xa61
(where a, b, c, and d denote respectively the atomic ratios of Al, Cr, Si, and B, and e denotes the atomic ratio of N. This is to be repeated in the following.)
The present invention includes preferred embodiments in which the value of e is 1, or the values of a and b are in the range of
0.02xe2x89xa61xe2x88x92axe2x88x92bxe2x89xa60.30, 0.55xe2x89xa6axe2x89xa60.765, 0.06xe2x89xa6b, or
0.02xe2x89xa61xe2x88x92axe2x88x92bxe2x89xa60.175, 0.765xe2x89xa6a, 4(axe2x88x920.75)xe2x89xa6b.
According to the present invention, the hard film for cutting tools should preferably be one which has the crystal structure mainly of sodium chloride structure. The sodium chloride structure should preferably be one which has the (111) plane, (200) plane, and (220) plane such that the intensity of diffracted rays from them measured by X-ray diffraction (xcex8-2xcex8 method), which is denoted by I(111), I(200), and I(220), respectively, satisfies expression (1) and/or expression (2) and expression (3) given below.
I(220)xe2x89xa6I(111)xe2x80x83xe2x80x83(1)
I(220)xe2x89xa6I(200)xe2x80x83xe2x80x83(2)
I(200)/I(111)xe2x89xa60.1xe2x80x83xe2x80x83(3)
In addition, the sodium chloride structure should preferably be one which, when measured by X-ray diffraction (xcex8-2xcex8 method) with Cu Kxcex1 line, gives the diffracted ray from the (111) plane whose angle of diffraction is in the range of 36.5xc2x0-38.0xc2x0. Moreover, the diffracted ray from the (111) plane should preferably have a half width not larger than 1xc2x0.
The above-mentioned hard film for cutting tools can be used to obtain coated cutting tools with outstanding wear resistance.
The present invention is directed also to a process for forming the above-mentioned hard film for cutting tools. This process consists of vaporizing and ionizing a metal in a film-forming gas atmosphere and converting said metal and film-forming gas into a plasma, thereby forming a film. The process is an improved arc ion plating (AIP) method which consists of vaporizing and ionizing a metal constituting a target by arc discharge, thereby forming the hard film of the present invention on a substrate, wherein said improvement comprises forming the magnetic lines of force which:
a) are parallel to the normal at the target""s evaporating surface, and
b) run toward the substrate in the direction parallel to or slightly divergent from the normal to the target""s evaporating surface, thereby accelerating the conversion of film-forming gas into a plasma by the magnetic lines of force.
In this case, the bias voltage to be applied to the substrate should preferably be xe2x88x9250V to xe2x88x92400V with respect to earth potential. In addition, the substrate should preferably be kept at 300-800xc2x0 C. while film is being formed thereon. The reactant gas for film forming should preferably have a partial pressure or total pressure in the range of 0.5-7 Pa.
Incidentally, the reactant gas used in the present invention denotes any one or more of gaseous nitrogen, methane, ethylene, acetylene, ammonia, and hydrogen, which contain elements necessary for coating film. The reactant gas may be used in combination with a rare gas such as argon, which is referred to as an assist gas. The reactant gas and the assist gas may be collectively referred to as a film-forming gas.
The present invention is directed also to a target used to form hard film which is composed of Ti, Al, Cr, Si, and B and has a relative density higher than 95%. The target should preferably contain no pores or pores with a radius smaller than 0.3 mm.
The target should have a composition defined by
(Ti1xe2x88x92xxe2x88x92yxe2x88x92zxe2x88x92w, Alx, Cry, Siz, Bw) 0.5xe2x89xa6xxe2x89xa60.8, 0.06xe2x89xa6y, 0xe2x89xa6zxe2x89xa60.1, 0xe2x89xa6wxe2x89xa60.1, 0xe2x89xa6z+wxe2x89xa60.1, x+y+z+w less than 1
(where x, y, z, and w denote respectively the atomic ratios of Al, Cr, Si, and B. This is to be repeated in the following.) In addition, if the value of (z+w) is 0, the values of x and y should preferably be in the ranged defined below.
0.02 less than 1xe2x88x92xxe2x88x92yxe2x89xa60.30, 0.55xe2x89xa6xxe2x89xa60.765, 0.06xe2x89xa6y, or
0.02xe2x89xa61xe2x88x92xxe2x88x92yxe2x89xa60.175, 0.765xe2x89xa6x, 4(xxe2x88x920.75)xe2x89xa6y.
Moreover, the target should preferably contain no more than 0.3 mass % oxygen, no more than 0.05 mass % hydrogen, no more than 0.2 mass % chlorine, no more than 0.05 mass % copper, and no more than 0.03 mass % magnesium.